How Can Cows Digest Cellulose?

Last Updated: 15.09.19

Humans use up to 98% of cattle for personal consumption and manufacturing additional products, and this is why the cow-growing industry is one of the most profitable worldwide. However, it takes passion and dedication to use the best cattle ear tagsand mark each cow at birth, as well as a large cattle feed troughto properly feed and look after your herd.

Cows are extremely interesting creatures and their eating habits and digestion process are also fascinating. For instance, as opposed to humans, cows can eat pretty much everything plant-based, including grass. Now, humans are omnivorous creatures, meaning they should also be able to survive solely on vegetarian diets.

Although this is true and we can indulge in various fruits, veggies, and even plants, eating grass like other ruminants won’t do us good. So, what’s the difference?

Why can’t humans digest cellulose?

Cellulose represents a complex carbohydrate that is found in the plant’s cell walls. Cellulose is present in all plants, including grass, kale, spinach, lettuce, and others. So how come we can eat some veggies but we cannot eat grass?

It’s not that we cannot eat grass but more that we cannot properly digest it and enjoy its nutrients. All other veggies and plants rich in cellulose, including spinach and kale, also include certain ingredients that human stomachs can break and digest. However, the grass contains lots of cellulose and our stomachs don’t have the necessary enzymes to break this ingredient.

Our digestive systems aren’t built or prepared for it since we also need certain enzymes to help us process meat and animal-origin products, apart from fruits, veggies, and certain plants.

What about the digestive system of cows?

Cows are part of the animal class of ruminants, together with sheep, buffalos, and goats. As opposed to other animals, the cow’s stomach consists of four separate chambers, each one being required for a certain digestive process.

When the cow starts eating grass, it goes down the esophagus, a canal connecting the throat to the stomach. After the grass is partly digested in the stomach, it goes down to the second chamber of the stomach named reticulum. Here, the grass is turned into cuds, which are basically chunks.

Following the forming of cuds, the animal will start its regurgitation process. The chunks previously formed in the second chamber of the stomach are pushed back to the mouth so the cow can chew on them a bit more and break them down piece by piece. After that, the ruminant swallows the grass chunks one more time and sends them back to the stomach.

The entire process takes hours so, the next time you see a cow indulging in some grass, learn that they aren’t slow eaters, their digestive systems only take more time to break down the cellulose in the grass, a process that humans are not able to do. In fact, goats and sheep have similar digestive systems and pretty much follow the same steps when eating.

How does cellulose digestion happen?

As we previously mentioned, cows and other ruminants have a stomach consisting of four different chambers. The first and the biggest one is called the rumen and is responsible for the actual digestion of the grass.

The rumen has small microbes or bacteria that are responsible for breaking down the cellulose contained in the grass and digesting it without requiring oxygen. The process is called anaerobic digestion and includes two different steps – the production of the required enzymes and the fermentation.

In the first phase of the process, the enzyme production, the microbes found in the first chamber of the stomach release different enzymes that will help with the digestion of cellulose. These enzymes act as catalysts, splitting the cellulose into smaller carbs like glucose.

After that, the resulted smaller carbs pass onto the third chamber of the stomach, the omasum, where they are fermented and absorbed as necessary nutrients.

The digested grass finally finds its way to the fourth chamber, named the abomasum. This is the acidic part of the ruminant’s stomach and is similar to the one in humans. Here, the remained food is digested a little bit more until it passes to the small and large intestines.

Basically, the reason why cows and other ruminants are able to digest cellulose found in the grass is because of the contained microbes that are responsible for the production of necessary enzymes. The human stomach lacks these enzymes but, instead, contains others designed to digest other types of carbs, including simple sugars and starch.

Even if you would put the same microbes found in the stomach of a ruminant inside the human stomach this process still wouldn’t take place because the latter has a higher acidity. The pH of a cow’s rumen is around 7, close to neutral, whereas the human stomach pH is somewhere between 1 and 3. The microbes won’t break down cellulose at a pH lower than 5.5, meaning that even if we were given the necessary enzymes, we would still lack the ability to get nutrients from grass.

What about other animals?

Over millions of years of evolution, animal species have adapted to different climates and conditions in order to survive. Other species like tapirs, horses, and rhinoceroses depend on a less efficient form of ruminant digestion, hence they are only considered distant cousins of ruminants.

Their digestive systems contain the necessary bacteria to break cellulose but the fermentation process mainly occurs in the intestine as opposed to the stomach. As a result, these animals won’t be able to extract all necessary nutrients from the plant, so that they cannot enjoy a healthy diet based solely on hay or grass.

However, in a fascinating example of convergent evolution, some monkey species are known for their leaf-oriented diet. After years of independent mutations, these mammals have developed a lysozyme enzyme with similar properties to the ones found in ruminants. Therefore, they are able to break down cellulose and benefit from the resulting nutrients.